This invention relates to knock sensors for use with multicylinder internal combustion engines. Such sensors are especially useful as part of a closed loop knock control system for such engines in which the ignition timing or some other engine operating variable is controlled in response to the output signal of such sensor to prevent excessive engine knock. Such sensors generally utilize magnetostrictive or piezoelectric elements combined with inertial masses to convert vibration induced strains on the elements into electrical output signals. Such sensors are generally mounted on an engine component in order to vibrate therewith and thus generate output signals representative of engine vibrations, including knock-induced vibrations.
Most such knock sensors known in the prior art have proven unsatisfactory for use in a practical knock control system on mass produced automotive engines. Some of such sensors are broadband sensors, in which output signal strength is substantially independent of vibration frequency over a very wide range of such frequencies. Such sensors are generally far too expensive for practical use on a mass production basis and are more suitable for experimental purposes in laboratories. In addition, the wide frequency response of broadband sensors results in too much output information at frequencies outside the general range of frequencies characteristic of knock-induced vibrations with a consequently poor signal-to-noise ratio, which can only be improved with electronic filtering. Since multicylinder internal combustion engines tend to generate significant amounts of vibrations other than those associated with knock, the signal-to-noise ratio of a knock sensor output signal generally requires as much improvement as possible.
One way, well known to those skilled in the art, of improving the signal-to-noise ratio of a knock sensor is the use of a sensor design with a mechanical resonance at a frequency associated with knock-induced vibrations to boost the signal strength of vibrations at that frequency relative to vibrations at other frequencies. In the knowledge of the inventor, this has always been done, with one exception to be described below, with a self-resonant sensor: that is, a sensor which resonates at a constant resonant frequency regardless of differences in the mass or other characteristics associated with the different modes of vibration of the object upon which it is mounted. A typical example of such a sensor uses a piezoelectric element mounted on a pedestal which is in turn fixed to a mounting stud for attachment to an engine component. A case, also attached to the mounting stud, may surround the pedestal and piexoelectric element, but is designed so as not to affect the resonant frequency of the pedestal. Such a sensor may be designed with a sharp, high "Q" resonance at a predetermined frequency and will provide a signal with a reasonably high signal-to-noise ratio at that frequency. Unfortunately, the frequency range of such a device is generally too narrow for practical application with a multicylinder internal combustion engine, since each cylinder of the engine may have its own characteristic knock frequency. The knock frequency of each cylinder is determined by the specific components associated with that cylinder; and since said components are similar from one cylinder to another, the individual knock frequencies tend to lie within a certain range of frequencies. However, there are slight differences in the normal manufacture of such components or design of the engine from cylinder to cylinder which often produce characteristic resonant frequencies in different cylinders of the same engine which are too far apart to be contained within the response of such a high "Q", self resonant knock sensor. If the response of the sensor is damped sufficiently to broaden the frequency range, the "Q" of the sensor, or the ratio of resonant to nonresonant amplitudes, is generally reduced so much that the sensor provides very little signal-to-noise ratio improvement.
There is one prior art knock sensor within the knowledge of the inventor which has proven practical for mass production use with multicylinder internal combustion engines. This sensor is produced by the assignee of this invention and used, at the time of the filing of this patent application, with certain turbocharged V-6 engines used on some vehicles also produced by the assignee of this invention. The sensor is a magnetostrictive sensor in which a magnetostrictive element axially in line with a mounting stud is subjected to an axial load through the case, which load varies with axial vibrations thereof. Electrical current through a coil surrounding the magnetostrictive element is thus caused to vary with such vibrations. This inventor has found that this particular sensor is not self-resonant: that is, although it exhibits a resonant frequency when mounted on an object, that resonant frequency changes over a range of frequencies in a predictable manner as the mass or equivalent mass of the object to which it is attached is varied--the sensor and object exhibit "interactive resonance". When that object is the intake manifold of a multicylinder internal combustion engine, this sensor exhibits a resonant response which combines a high ratio of signal output in the resonant region to signal output in the nonresonant region with a resonant frequency response that is significantly wider than that of a self resonant sensor. The frequency range of the former resonant response is sufficiently wide to cover at least the cylinder to cylinder variations of characteristic knock frequency associated with the V-6 engine on which it is used.
However, although this sensor is significantly less expensive than the aforementioned broadband sensors, it is still sufficiently complex in its structure to make a less expensive knock sensor desirable for mass production use. In addition, this sensor responds mainly to vibrations parallel to the axis of its mounting stud and thereby may ignore significant knock information which might exist in the form of vibrations transverse to this axis. Therefore, a less expensive knock sensor which is interactive in resonance with an engine component on which it is mounted and responds to transverse as well as axial vibrations would be desirable. Such a sensor utilizing a piezoelectric element shows promise of being simpler in structure than a magnetostrictive sensor and therefore perhaps less expensive to manufacture. In addition, the higher amplitude output of a piezoelectric device provides further advantages.